This project is mainly concerned with characterizing several naturally occurring slow viral diseases of domestic animals that resemble important, etiologically obscure diseases of man. Two diseases receive most attention: scrapie of sheep and goats and Aleutian disease of mink. Their clinico-pathologic, virologic, and immunologic features are studied to provide an understanding of their pathogenesis and natural history. Information from such studies will help define the unusual host-virus relationships that result in protracted disease in man and animals.